Tree Lighting

December 1, 2018

City Hall will be Closed December 25, 2018 for
Christmas

2017 Solar Eclipse

The August 21, 2017 solar eclipse will be the first total solar eclipse in the continental United States since 1979, and the first to sweep the country since 1918! The path of totality will be 60 miles wide, and will make first landfall just north of Depoe Bay at Government Point (Boiler Bay State Park).

Q. Since Oregon is on the west coast and the sun rises in the east, how can Oregon be the point of landfall when the sun will be behind us? Won't we have to face east - away from the ocean - to see it?

A: it is geometry that causes the effect you are speaking about. From our perspective the Moon appears to move in front of the Sun. In reality (as seen from Earth) the sun is moving behind the Moon. So as the two bodies shift the shadow appears to move across the surface of the Earth. So as those angles change, it give the appearance that the shadow is moving backwards across the Earth's surface

Let's describe a closed system that contains the Sun, the Earth, and the Moon. For all intents and purposes, in this system, the Sun is stationary. The two remaining objects (Earth and Moon) both move from West to East around the Sun. Put another way; if you could go 1000 miles straight up from the Earth's North Pole you would see that the Earth rotates counter clockwise (CCW). The Moon too orbits around the Earth in a CCW motion. The Moon's orbital speed around the Earth is a whopping 3,686 km/h. This is more than 2 times faster than the Earths rotational velocity (1,675 km/h at the equator). So since everybody (Earth and Moon) are moving West to East and the Moon moves (relatively speaking) 2 times faster than the Earth rotates, the Moon’s shadow moves from west to east along the path of totality.

Q: Why does the eclipse go from west to east, when the sun and moon go the other way?

A: Well, the movement of the Moon -- from East to West -- is, in fact, an illusion caused by the Earth's rotation. As a matter of fact, the Moon orbits in the same direction that the Earth rotates; anticlockwise, as seen from above the North pole. But whereas the Earth takes just 24 hours to do one rotation, the Moon takes a month to go round the Earth (actually, the Moon takes 27.32 days to orbit the Earth).

In other words, if the Earth was sitting still, the Moon would cross the sky from West to East. It would take 13½ days to cross from horizon to horizon, and another 13½ days to come around into view again. But the Earth doesn't sit still -- it rotates, every 24 hours, which is significantly faster than this. It's like if you're driving a car and overtake a jogger, they seem to be going backwards relative to you; the Earth rotates faster than the Moon's orbit, so the Moon seems to be going backwards, when it's actually going the same way.

So what happens to "fix" things during an eclipse? Well, the Moon orbits the Earth once a month; but the distance that it travels in that month is a whopping 2,415,256km! This means that it's moving really fast. By contrast, the Earth is a tiny 12,000km across; so for the Moon to cross in front of the Earth -- for its shadow to cross the Earth -- doesn't take long at all; the Moon moves 12,000km in just 3½ hours. (The exact time for the eclipse to cross the Earth depends on whether the Moon is crossing over the centre of the Earth or off-centre, and on what part of its elliptical orbit the Moon is in.) So the shadow zips across much faster than the Earth's rotation, which makes its real direction apparent.

To put it another way, the Moon only has to cross a tiny part of the sky -- a small fraction of its total orbit -- for its shadow to cross the Earth completely. This means that for an eclipse, the Moon's own "real" movement is the main cause of its movement; so the shadow goes West-to-East.